This invention relates to a flywheel. More particularly, this invention relates to a flywheel for ultra-high speed operation having a rim and a hub including radially extending spokes coupling the rim to a shaft.
Flywheels have long been recognized as convenient devices for the storage of mechanical energy. Energy is stored in a flywheel by causing it to rotate at a high speed about an axis of rotation defined by a shaft. By mounting the shaft in low-friction bearings and the flywheel in an evacuated chamber, frictional energy losses are minimized. Thus, the flywheel has come to be recognized as a convenient device for the relatively long-term storage of energy. Particular attention has been directed to the flywheel as a device for energy storage in mass-transportation vehicles operating under stop-and-go conditions. For example, the flywheel may be charged with energy by bringing its rotational speed to a high level while the vehicle is stopped. Energy is then drawn from the flywheel to accelerate the vehicle and power it toward its next stop. By using regenerative braking energy which would conventionally be dissipated as heat is returned to the flywheel for later use. Thus, the flywheel provides a conceptually simple means of storing energy for vehicular and other uses.
However, attempts to construct and utilize such a flywheel have been fraught with difficulties and failures. For example, because the energy stored in a flywheel varies directly with its moment of inertia and as the square of its rotational speed, very high operating speeds for the flywheel are desired. Further, in order to obtain best performance from a vehicle, the weight of the flywheel must be kept to a minimum to reduce vehicle weight. Therefore, flywheels having a rim have been recognized as offering the highest moment of inertia for a given weight. When the rim is made of a multitude of concentric annular shells made from circumferentially extending unidirectional filamentary material in a matrix it is well adapted to withstand the high stresses imposed by centrifugal force at high rotational speeds. Such flywheels conventionally have a hub including spokes coupling the rim to the shaft. U.S. Pat. Nos. 860,336; 3,724,288; 3,964,341; 4,036,080; 4,176,563; 4,183,259 and 4,186,623 illustrate such flywheels.
However, even though the stress imposed by centrifugal force varies with the square of the radius from the axis of rotation so that the rim is most highly stressed, at the ultrahigh rotational speeds desired the spokes are also highly stressed. Thus, the desirability of also making the spokes of high-strength, low-weight unidirectional filamentary material in a matrix has been recognized. U.S. Pat. No. 4,286,475 illustrates such a flywheel.
Unfortunately, at the ultra-high rotational speeds desired, the rim and spokes of a flywheel stretch and distort to expand radially. Even the unidirectional filamentary material embedded in a matrix is elastic and deforms when exposed to the high centrifugal forces created by ultra-high speed operation of a flywheel. Thus, the flywheel designer is left with the difficult problem of how to unite matrix material spokes to form of hub for the flywheel. U.S. Pat. No. 4,286,475 illustrates one solution to this problem.
In view of the many deficiencies of the flywheel art, it is an object for this invention to provide a flywheel having a rim and spokes coupled to a shaft by hub portions which distort in reponse to centrifugal force to match the distortion of the spokes.
Another object for this invention is to provide a hub for a flywheel having spokes substantially avoiding stress concentrations between the spokes and the remainder of the hub.
Still another object for this invention is to provide a hub for a flywheel with spokes of unidirectional filamentary material embedded in a matrix.
In summary, one embodiment of this invention provides a flywheel having a rim and spokes of filamentary material embedded in a matrix. The spokes engage the rim and extend radially inwardly toward but short of the axis of rotation. A hub includes portions axially coextensive with each spoke over a radially extending segment of the latter. The portions are adhesively bonded to the spokes and decrease in transverse cross sectional area with increasing radius throughout the radially extending segment. During operation of the flywheel, the portions distort in response to centrifugal force to expand radially substantially in unison with the spokes.
Other objects and advantages of the invention will appear in light of the following detailed description of a preferred embodiment of the invention.